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1
Light
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Sources of Light
Anything that produces light is called a light
source. Light sources can be natural, eg. the
sun, or artificial, such as light bulbs or
television screens.
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Travelling Light
Light travels from a light source in straight
lines. We call these light rays. When we draw
light rays, we must use a ruler, and we must
always draw an arrow on the line to show the
direction the light is travelling in, away from
the source.
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Travelling Light
Light passes easily through air. We say it is
transparent. If light hits an opaque object, it
cannot pass through. This causes a shadow, as the
light rays cannot bend around the object.
Some materials, eg. tracing paper, will let some
light through, but we cannot see through them
clearly as if we were looking through a
transparent material such as glass. We say these
materials are translucent.
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Why We See Things
We see things when rays of light from them enter
our eyes. The rays of light can travel directly
into our eyes if they are from a light source
such as from a light hulb. However, we see most
things because they reflect some light into our
eyes.
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The Eye and the Camera
The eye and the camera both work in a very
similar way. They each have a hole which allows
light rays in, and produce an inverted (upside
down) image.
Rays of light travel in all directions from the
head, but most of them cannot get into the
box. However, imagine a ray of light from the top
of the head that is heading towards the pinhole.
(Click).
Imagine another ray of light travelling from the
bottom of the head. This ends up at the top of
the image.
We end up with an image that is inverted.
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The Eye
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Reflection and Scattering
When light hits an object, some of it is
reflected. How much is reflected depends on the
colour (see later), and how rough or smooth the
surface is.
A mirror has a very smooth surface and is an
extremely good reflector. Most surfaces have a
rough texture, and they scatter the light in all
directions.
Road surafces are rough, but when the surface is
wet water fills in the holes and the smooth
surface becomes very shiny as it reflects a lot
more light.
Excellent interactive reflection demo
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The Law of Reflection
If we shine a ray of light at a mirror we can
predict how the ray will bounce off.
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The Law of Reflection
Ray box
Angle of incidence
I
Normal Line
R
90o
Angle of reflection
The angle of incidence is always equal to the
angle of reflection.
Reflected ray
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How a Periscope Works
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Bending Light - Refraction
Air
A ray of light changes direction when it goes
into a different substance. This is called
refraction.
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Bending Light - Refraction
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Bending Light - Refraction
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Fill a beaker with water, put your pencil in it
and look at the pencil from above, draw the
diagram below to show what you see and try to
draw in the light rays.
Bent Pencil
refraction
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The ray of light seems to come from here, this is
where you see the pencil
refraction
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Refraction At Work
A fish swimming underwater looks to an observer
above the water to be in a different position to
where it actually is. Remember the coin in the
cup trick!
Looking into water
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White Light?

• What we think of as white light is actually a
  mixture of all colours of light.
• Sir Isaac Newton decided that white light is made
  up of seven colours
• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet
• We now know that the human eye can distinguish
  millions of different colours and shades.

We can use a saying such as Richard of York
gave battle in vain ?Try to think of your own
saying to help you remember the colours of a
spectrum.
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Newton's prisms
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Prisms
Red Orange Yellow Green Blue Indigo Violet
White light
Dispersion of white light to produce a spectrum
occurs because some colours refract more than
others.
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Colour Filters
When white light is shone towards a colour
filter, not all of it is transmitted through the
filter.
The red light passes through the filter. All the
other colours are absorbed.
This time blue light passes through the
filter. All the other colours are absorbed.
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Mixing Colours
We can mix coloured light to form other colours.
Mixing light is very different to mixing
paints. The three primary colours are red, green
and blue. These are the only colours that a
television screen produces. The whole picture is
made up of red, green and blue dots! When mixed
in pairs these produce the three secondary
colours, cyan, magenta and yellow (these are the
three coloured inks you find in a colour printer).
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The Primary Colours
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Red Blue
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Red Green
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Blue Green
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Red Green Blue
Mixing Primary Colours
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Coloured Objects
A green object looks green because it reflects
green light and absorbs the other colours.
Can you explain these examples?
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Why do different objects look different colours?

• We see non-luminous objects because they reflect
  light into our eyes.
• ?RED objects look red becausethey reflect only
  RED light
• ?BLUE objects look BLUE because they
  reflect only BLUE light

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Remember!
Only RED light can get through a RED filter
Only BLUE light can getthrough a BLUE filter
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BLUE object and BLUE filter
BLUE objects reflect only blue light
So it looks BLUE
A BLUE filter lets blue light through
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RED object and RED filter.
So it looks RED
RED objects reflect only RED light
A RED filter lets RED light through
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RED object and BLUE filter
BLUE filters only let through BLUE light. The RED
light gets absorbed
RED objects reflect only RED light
No light gets here so the object looks BLACK
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1. Draw a diagram to show how we see a
non-luminous red object in white light, your
diagram should show why the object appears red.
The bulb produces white light.
The RED object reflects RED light into our eye,
all other colours of light are absorbed by the
object.
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• 2. What colour would a red object appear in red
  light?
• RED
• 3. What colour would a blue object appear through
  a blue filter?
• BLUE
• 4. Why do black objects appear black in white
  light?
• Because they absorb all colours of light.
• 5. What colour would a blue object appear through
  a red filter?
• BLACK

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You have now reached the end of the revision
presentation for the Light topic. I hope you
have found it useful. If you have any ideas to
improve this presentation please let Mr. Turner
know. Thank you for looking at this work, and
good luck in your test.
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